The development of catalysts for efficient conversion of methane to methanol has received increasing attention in recent years with the search for alternative fuels to replace the dwindling supply of petroleum. Recent advancements in natural gas exploration and production technology have led to a dramatic increase in both natural gas production and known natural gas reserves. Natural gas is composed primarily of methane mixed with other volatile hydrocarbons such as ethane, propane, butanes, pentanes and hexanes. Although natural gas is used as a primary combustible fuel, there is a substantial interest in technologies that are capable of converting natural gas into a more efficient and transportable fuel. One potentially attractive conversion pathway is oxidation of natural gas into oxidation products, such as alcohols, which have higher specific energies, lower volatility and more chemical versatility.
Methanol has a range of commercial applications including its use as a feedstock for the manufacture of industrial chemicals (e.g. formaldehyde, di methyl ether, etc.), as a fuel or fuel additive (e.g., automobiles) and as a solvent for synthesis and manufacturing applications. Currently, the most common process for the synthesis of methanol is the reaction of carbon monoxide with hydrogen gas using a copper-based catalyst. This process is endothermic, however, and requires both high temperature (˜250° C.) and pressure (50-100 atm). Natural gas or pure methane can be used as a feedstock to generate carbon monoxide and hydrogen, typically by reacting methane with steam using a nickel catalyst, which can be subsequently used to generate methanol. Again, however, this reaction is endothermic and requires high temperature (˜850° C.) and high pressure (˜40 atm). Thus, many conventional methods for converting methane to methanol are resource intensive and costly in that these methods require multiple steps, expensive catalysts and significant input of energy.
Direct conversion of methane to methanol, a gas-to-liquid process, is challenging because the C—H bond in methane has a very high bond-energy (104 kcal/mole) and, thus, is largely inert chemically. In addition, the methanol reaction product of this reaction is prone to further oxidation under many conditions, thereby make selective generation of methanol at high yields difficult. Similarly, the oxidation of short length alkanes other than methane to their corresponding alcohols is also challenging as the C—H bond energies in these molecules are just slightly lower than the C—H bond energy of methane, with similar problems of over-oxidation.
Methanotrophic bacteria, such as Methylococcus capsulatus and Methylosinus trichosporium, are capable of the facile conversion of methane to methanol even at ambient temperatures and pressures. These bacteria mediate oxidation of methanol by using the enzyme methane monooxygenase (MMO) and dioxygen [See, e.g., Chan, Sunney et al. (2004) Toward delineating the Structure and Function of the Particulate Methane Monooxygenase from Methanotropic Bacteria. Biochemistry, 43 (15), 4421-4430.] There are two different forms of MMO: particulate (pMMO) and soluble (sMMO). In particular, the pMMO enzyme exhibits amazing regiospecificity and stereoselectivity in the oxidation of straight-chain alkanes from C1 to C5.
Researchers have studied the biochemical reactions in which pMMO facilitates the conversion of alkanes to alkanols with a potential goal of producing a biomimetic catalyst capable of oxidizing hydrocarbons under ambient conditions. As a result of this research, consensus has begun to develop supporting an understanding that the enzyme incorporates a multicopper cluster as an active site for oxidation [See, e.g., Chan, Sunney et al. (2008) Controlled Oxidation of Hydrocarbons by the Membrane-Bound Methane Monooxygenase: The Case for a Tricopper Cluster, Accounts of Chem. Research, 41 (8), 969-979.] Such research has also demonstrated that enzymes such a sMMO and pMMO require tight kinetic control in order to efficiently regenerate the catalyst back to the precursor state, presenting a further challenge for development of a viable biomimetic catalyst. The need for kinetic control can be potentially circumvented in transition metal catalysts, however, by operating a metal ion at a higher oxidation state or by resorting to Fenton-type chemistry [See, e.g., Chan, Sunney et al. (2012) Efficient catalytic oxidation of hydrocarbons mediated by tricopper clusters under mild conditions, J. Catal., 293, 186-194.] Previous efforts to develop functional biomimetic catalysts based on sMMO or pMMO have met with limited success.
PCT Publication No. WO 2011/035064 A2 to Elgammal, for example, is directed to a catalyst for the oxidation of hydrocarbons comprising a 1,2,4-triazole ligand and a transition metal. The catalyst operates at temperatures between 0-25° C. The yield of the oxidation of methane to methanol, however, remains under 30%, and in most cases much lower. Further, in reactions to oxidize larger alkanes, low regiospecificity and over-oxidation both remain major problems.
It will be appreciated from the foregoing that there is currently a need in the art for an improved molecular catalyst and methods capable of the facile oxidation of hydrocarbons at ambient conditions with a high degree of regiospecificity and stereoselectivity. In particularly, catalysts are needed that provide catalytic oxidation of hydrocarbons from natural gas exhibiting high turnover frequencies and specificity for formation of useful oxidation products, such as methanol.